The present invention relates to digital video processing and more particularly to a method for applying film grain and texture mapping to a digital video image.
The photographic film used to make motion pictures comprises silver-halide crystals dispersed in an emulsion that is deposited in thin layers on a film base. The exposure and development of these crystals form the photographic image, which is made at the largest scale of discrete particles of silver. With color negatives, tiny blobs of dye exist on the sites where the silver crystals form after chemical removal of the silver following development. These small specs of dye form the ‘grain’ in color film. Grain occurs randomly in the resulting image because of the random formation of silver crystals on the original emulsion. Within a uniformly exposed area, some crystals develop by exposure, others not. Grain varies in size and shape. The faster the film (i.e., the greater the sensitivity to light), the larger the clumps of silver formed and blobs of dye generated, and the more they tend to group together in random patterns. The grain pattern is typically known as ‘granularity’.
The naked eye cannot distinguish individual grains, which vary from about 0.0002 mm to 0.002 mm. Instead, the eye resolves groups of grains, which the viewer identifies as film grain. The larger the image resolution, the more likely the viewer will perceive the film grain. In some sense, film grain constitutes the correlated noise inherent in the physical process of developing of photographic film. The presence of film grain denotes ‘real-world’ images in contrast to ‘computer-generated’ material with no film grain. Images without film grain look like hyper-reality, simply too flat and too sharp to be real. Film grain adds a layer of dithering which replicates the human perceptual system. For this reason, film grain remains a desired ‘artifact’ in most images and video material.
However, it is becoming increasingly difficult to acquire motion picture film raw stock and to get the film stock processed in a timely manner at a laboratory. Accordingly, digital video is being used for more television shows and movies. Because digital video does not have film grain, methods have been developed to try and impart a film grain look to digital video
Currently, the methods for making digital video look like film generally fall into two categories: applying digital film grain or applying analog film grain. The first method (digital grain) consists of computer generated video noise designed to replicate the random particles of film grain and create the effect of “film grain”. This method is only moderately successful in that it is able to create a seemingly random pattern of noise, but that pattern of noise looks “digital” rather than analog. The grain is mostly of the same size, texture and color. When film is developed, it goes through several chemical baths that create patterns, geometries, waves and colors in the film emulsion itself. These are very subtle but noticeable effects that fail to be rendered by digital grain. While in some instances digital grain might look somewhat “film like” on a small screen, the digital artifacts created by digital grain and the lack of true movement, geometry and patterns do not survive scrutiny on a large theatrical motion picture screen.
The second method (analog film grain) usually involves simply scanning a piece of motion picture film and overlaying the grain of the film over the video image. While better than digital grain, this process produces only mediocre results and has at least two major inherent flaws. First, scanning a single piece of film does not accurately represent the full tonal value and granularity of a particular motion picture film stock. One singular area of the original film stock is used to grossly represent the entire stock. This over-simplification results in a flat, two-dimensional representation of the original film stock that makes it appear more like digital grain than analog grain due to the repetitiveness of the shapes and geometry over the entire image area. Second, the application of film grain onto digital video is presently mostly a post production rendered process. This means that the digital video project is first completed, including full color correction, and then a layer of film grain is applied and rendered to your footage for later review and approval. This method does not allow for creativity and input from filmmakers at the color correction stage. It is simply an “apply and bake” process with very little control and finesse.
Accordingly, there is a need for an improved system and method for imparting film like grain to a digital video image that remedies the shortcomings of the prior art.